1. Field of the Invention
The present invention relates to a tire information detector, and more particularly, to a tire information detector for detecting tire information such as pressure of a tire used in a vehicle.
2. Description of the Related Art
In the related art, a radio transmission apparatus transmitting a measurement value such as pressure of a tire used in a vehicle to a controller provided in a vehicle body in a radio manner, and for example, evaluating the measurement value for an alarm message to a driver has been proposed. An example of the related art is disclosed in Japanese Examined Patent Application Publication No. 3494440, FIGS. 3 and 5. In such a radio transmission apparatus, a controller as shown in FIG. 4 is provided in a vehicle body and a measured-value transmitter (transponder) as shown in FIG. 5 is provided in the tire.
As shown in FIG. 4, the controller includes a carrier wave oscillator G1 generating a carrier wave (f1) of about 2.4 GHz, a modulator MO1, and a oscillator (modulation wave oscillator) G2 outputting an excitation signal. The oscillator G2 outputs an excitation signal with a frequency (f2) close to a resonance frequency of a resonator of a transponder to the modulator MO1. The carrier wave from the carrier wave oscillator G1 is modulated in amplitude by the excitation signal from the oscillator G2, a high frequency signal of 2.4 GHz modulated in amplitude is amplified by an amplifier (not shown), and then the signal is emitted from an antenna A1.
The controller includes a switch S1 determining modulation or non-modulation in amplitude by the modulator MO1, a receiver E1 receiving the high frequency signal emitted from the transponder and calculating a measurement value (S1) such as the pressure of the tire, and a timer T1 controlling a switching time of the switch S1 and a state of the receiver E1. The modulation or non-modulation in amplitude of the carrier wave is determined by the timer T1, the high frequency signal modulated in amplitude is transmitted for a predetermined period, and then the modulation in amplitude stops at a point of time t1 to transmit the non-modulation carrier wave. The receiver E1 is activated at the point of time t2 within about 1 μs after the point of time t1 and receives the high frequency signal from the transponder through an antenna A4.
As shown in FIG. 5, the transponder includes low pass filters L11/C11, a diode D11 serving as modulator/demodulator, a capacitive pressure sensor (hereinafter, referred to as ‘pressure sensor’) SC11 in which capacitance varies depending on the pressure of the tire, and a resonator having a crystal resonator Q11 excited by the excitation signal of the high frequency signal from the controller. The excitation signal is extracted from the high frequency signal from the controller by the low pass filter L11/C11 and the high frequency signal is modulated by the diode D11. In this manner, the excitation signal of the oscillator G2 is extracted. Since the resonance frequency of the resonator is close to the frequency of the excitation signal of the oscillator G2, the resonator is excited by the excitation signal. The resonance signal of the resonance frequency is generated by the excitation. When the capacitance of the pressure sensor SC11 varies depending on the pressure of the tire, the resonance frequency of the resonator varies. Accordingly, the resonance frequency of the resonance signal is affected by the variation.
As described above, the controller transmits the high frequency signal modulated in amplitude, stops the modulation in amplitude, and transmits the non-modulation carrier wave even when the modulation in amplitude stops. The resonator continuously oscillates about 1 ms or more. For the reason, the non-modulation carrier wave from the controller is modulated in amplitude by the diode D11 based on the resonance signal of the resonator and is emitted from an antenna A3. The receiver E1 receives the high frequency signal modulated in amplitude through the antenna A4 and extracts the resonance signal through a demodulator (not shown), thereby calculating a measurement value S1 such as the pressure of the tire.
However, in the above-mentioned radio transmission apparatus, the resonance frequency of the resonator of the transponder sequentially varies and the crystal resonator Q11 is used as the resonator. Accordingly, it is possible to perform the communication in which the Q value of the resonator is greatly stabilized. On the contrary, since the band of the resonance frequency is narrow, a usable frequency band of the excitation signal becomes narrow. When the frequency of the excitation signal of the oscillator G2 is out of the resonance frequency of the resonator, a response of the resonator becomes small. As a result, it is difficult to accurately detect the tire information such as the pressure of the tire. The same problems occur even when the resonator is not the crystal resonator. However, since the crystal resonator has high Q value, the problems become prominent.
In order to cope with the above-mentioned problem, in the known radio transmission apparatus, the frequency of the excitation signal generated by the oscillator G2 is slightly varied in a predetermined range to be outputted to the transponder so as to approximate the frequency of the excitation signal to the resonance frequency of the resonator, and the response of the resonator is determined, thereby adjusting the frequency of the excitation signal. However, in this case, the time for adjusting the frequency of the excitation signal is required and it takes a long time until the tire information such as the pressure of the tire is detected.